The present invention relates to an image forming apparatus, for example, a color image forming apparatus in which an electrostatic latent image is formed on an image carrier using a laser beam source, and a plurality of developing processes are repeatedly conducted using color toners, and more particularly relates to an image forming apparatus in which the fixing condition is changed in the process of color image formation in order to form a desired color image accurately.
The change of the fixing condition will be described as follows. In a monocolor or full color image forming apparatus, toner images are transferred onto a transparent resin film (usually referred to as an OHP sheet) for use in an overhead projector. In the case of an OHP sheet, light permeability is a requisite. However, irregular reflection is caused on the surface of the OHP sheet, so that the image tends to become black when the same fixing condition as that used for a regular paper is adopted. Therefore, the OHP sheet must be heated to a higher temperature than that of the regular paper in order to improve the surface property of a toner layer. This can be realized when the temperature of the fixing unit is raised or the feed speed of the transfer sheet is reduced. On the other hand, in the case of a regular paper, it is not desirable to a heat the paper to high temperature or to reduce the feed speed. Therefore, the fixing condition of the regular paper and that of the OHP sheet must be changed. However, when the fixing condition is changed, the grade of deformation of a dot and line is varied. Consequently, in the case of the OHP sheet, the line width and dot diameter are increased.
When the OHP sheet is subjected to a usual fixing process in which the transfer sheet is heated to a standard fixing temperature (about 180.degree.), the transmitting property is low since the smoothness of the OHP sheet surface is low. Therefore, when the feed speed is lowered to 1/6, the toner layer is sufficiently fused into the OHP sheet, so that the permeability can be improved. However, this low speed is disadvantageous in that the dot portion of the toner image becomes bold and the line also becomes bold.
As an example of image formation in which the fixing condition is changed, the following case can be considered: in order to obtain a desired glossy image, the fixing condition is changed. In this case, an excellent image can not be provided only when the fixing condition is changed.
Therefore, the first object of the present invention is to obtain an excellent toner image in the following manner: after a toner image has been transferred onto a transfer sheet, the transferred toner image is fixed so that the dot diameter and line width of the fixed image are not varied by fixing conditions such as the state of fusion and fixing speed.
A conventional electrophotographic color image forming method by which color images are formed when color toner images are superimposed, will be explained as follows. At first, as shown in FIG. 1(a), image carrier E2, on which a color toner image is formed, is charged for the first time by a charging electrode E1 so that the surface potential of the image carrier can be V.sub.S. Next, only a portion of an image on which a character or a line image is formed is exposed by a laser beam as shown in FIG. 1(b). After the exposure has been completed, only the exposed portion is discharged and the surface potential V.sub.S is lowered, and non-image portions are not discharged. Next, as shown in FIG. 1(c), the image carrier E2 is moved in the arrowed direction and the first color development is conducted by a developing unit D. In this case, the electrical charge is eliminated from the exposed portion on image carrier E2 by the method of "well type electrical charge erasing" as shown in FIG. 1(b), and the first color toner is adhered onto the eliminated portion, and then the first color toner is developed so that the first color toner image is formed. Next, the second charging is conducted by charging electrode E1 on the image carrier surface including the first color toner image portion, as shown in FIG. 1(d). In FIG. 1(d), for the convenience of explanation, the first color toner image is illustrated at the position of electrical charge V.sub.S, separated from the surface of image carrier E2. Next, as shown in FIG. 1(e), when the second exposure is conducted on the first color toner image, the electric charge can be eliminated from the exposed portion on the image carrier by the method of "well type electrical potential erasing" in the same manner as shown in FIG. 1(b). Then, the second color toner is developed by developing unit D so that the second color toner image is superimposed on the first color toner image. The foregoing is a basic process in which a color image is formed on the image carrier when color toner images are superimposed on each other. The specific means of the aforementioned color toner forming means is as follows: as shown in FIG. 2, a display 100 is used in which a computer (referred to as a CPU, hereinafter) is provided; data is inputted into the CPU through a key inputting means mo be stored by a magnetic recording means; and the recorded characters, lines and solid images are outputted into a printer controller 101. From the printer controller 101, dot images, for example, character A and the like are successively outputted, and the dot signals are inputted into a laser printer 102, and then a toner image is formed by the laser printer 102 through the processes of (a), (b), (c), (d), (e) and (f) illustrated in FIG. 1.
In order to obtain a color toner image by the aforementioned color toner superimposing system, there are 2 exposure methods, one is a pulse control method, and the other is a laser beam intensity control method. For example, when red is made from yellow and magenta, the color tone must be delicately adjusted in the following manner: yellow is strengthened and magenta is weakened; and on the contrary, yellow is weakened and magenta is strengthened. In order to control the color tone and obtain a desired color toner image, the exposure condition is changed so that the mixing ratio of color toner images can be changed. In the aforementioned color image forming process, when the second color exposure is conducted on the first color toner image, the first color toner layer is greatly affected by the second color exposure. Accordingly, the aforementioned color toner image forming method is disadvantageous in that it is difficult to appropriately control color toner development.
The aforementioned color toner image forming method has the following disadvantages. As shown in FIG. 3, the image signal outputted from the CPU to the printer controller 101 includes a dot image signal representing a character, a line signal and a solid image signal. First, FIG. 3 is explained as follows. A pulse-like electrical image signal is inputted into the printer controller 101 as shown in FIG. 3(b). In FIG. 3, signals are successively inputted from the left, and first, a dot image signal is inputted. A little after that, a solid image signal is inputted. These input signals are outputted to the laser printer 102 from the printer controller 101 in the form of a video signal.
The above-described video signal is converted into pulse signals by a pulse modulation circuit in the laser printer 102, and an ON signal corresponding to a dot image output signal is generated and a pulse having a constant width is formed. Next, a solid image video signal is inputted into the laser printer 102 after an interval, and when a pulse signal having a constant width and a constant interval is turned on, the solid image output signal is outputted. When this is illustrated by the light energy intensity as shown in FIG. 3(a), for example, when a duty ratio of the exposure unit is adjusted to 50% in order to limit an adhered amount of the first color, and it is adjusted to 100% for the second color, at the time when a distribution of the optical energy density which is received by the photoreceptor, is illustrated, a large difference of the energy density is generated between the 50% duty ratio in the case of a dot image, and the 50% duty ratio in the case of a solid image. That is, in the case of the solid image, an energy amount is a total of the energy of dots which are close in position to each other. Therefore, an average energy of the solid image is larger than that of an independent isolated dot, one dot line, or an edge portion of the solid image. Accordingly, the difference of the optical energy of the solid image portion between the case of 50% duty ratio and the case of 100% duty ratio, is reduced as shown in FIG. 3(a). In this condition, when the surface potential curve in which the image carrier shown in FIG. 1 is exposed, is shown in FIG. 3(b), the difference of the surface potential of the dot image between the case of 50% duty ratio and the case of 100% duty ratio is VS.sub.1, which is large. That is, the surface potential in the case of 100% duty ratio is lowered by VS.sub.1 than that in the case of 50% duty ratio
In contrast to such a condition, even when the electric charge is eliminated by the image exposure of the solid image in the case of the 50% duty and in the case of the 100% duty, the difference of the surface potential of the solid image is VS.sub.2, as shown in the drawing, which is very small. After the above-described charging, and image exposure processing, and developing processing are conducted, when the image forming processing shown by FIGS. 1(c), (d), (e), (f) is conducted, a condition of the electric charge elimination by the exposure in the 50% duty ratio, is shown in FIG. 3(c). In the dot portion and the solid image portion, since the potential drop of the dot portion is small, when developing is conducted by the developing unit D as shown in FIG. 4(a), an adhered amount of toner is very small, and since the potential drop of the solid image portion is large, toner is fully adhered to the portion. Next, charging is conducted again on the toner image in the first color development as shown in FIG. 4(b), and the second exposure of the 100% duty is conducted for the second color development as shown in FIG. 4(c). In general, when the second exposure is conducted after the first toner image has been formed on image carrier E2, it is necessary to increase light intensity since the second exposure is conducted through the first toner image. For that reason, light energy of duty 100% is projected on both the dot portion and the solid image portion. In this case, the toner adhesion amount of the dot portion on image carrier E2 is small in the first color toner image development, so that a large amount of light energy is projected to the dot portion. Accordingly, the electrical charge on the dot image portion is greatly reduced. On the other hand, the toner adhesion amount of the solid image portion on image carrier E2 is large, so that a small amount of light energy is projected. Consequently, as shown in the drawing, erasing of electrical charge in the solid image portion is smaller than that in the dot portion. AS described above, there is a large difference between the attenuation ratio of the dot portion and that of the solid image portion. In this case, when the second color toner image is developed by developing unit El, a large amount of toner is adhered onto the dot portion since a large amount of electrical charge is erased in this portion. Therefore, the adhesion amount of the second color toner in the dot portion is far larger than that of the first color toner, so that it is difficult to adjust colors. However, in the case of the solid image portion, the toner adhesion amount of the first color and that of the second color are uniform so that a desired toner image can be obtained. As explained above, when the same charging operation is conducted on the dot and solid image portions and further the exposure of the same light intensity is conducted, it is impossible to obtain excellent color tone.
The present invention has solved the aforementioned problems. The second object of the present invention is to solve the following problem: when a color image is formed by superimposing a plurality of color toner images according to the image information of a character, dot, line and solid image outputted from the CPU image memory, the second color toner can not be adhered onto the image carrier in the same manner as the first color toner; and desired color reproduction can not be carried out. It has been found that there is a difference between the toner adhesion amount of the central portion of an image and that of the edge portion, so that problems are caused in color reproduction. The present invention is to reproduce a desired color image in this manner: when an image signal provided to the exposure means is modulated after the aforementioned image information has been inputted into the image forming apparatus in which a photoreceptor is adopted, a desired controlling operation is conducted on the exposure means.
The abovementioned first object can be accomplished when the control means with regard to the exposure means is utilized. That is an excellent image can be provided, irrespective of the change of the fixing condition, when exposure means is controlled rather than correcting the fluctuation of a fixed image caused in the case of change of the fixing condition.